The present invention relates to a method and system for disrupting the flow from the smelt spout secured to the wall of a recovery boiler in a cellulose pulp mill (e. g. a kraft pulp mill), utilizing at least two properly positioned nozzles for introducing a disrupting medium for breaking up the smelt discharged from the boiler via the spout into a dissolving tank placed under the spout.
An essential apparatus in the chemical recovery circulation of sulfate and other sodium-based pulp manufacturing processes is the recovery boiler, such as a soda recovery boiler. In the recovery boiler waste liquor (e. g. black liquor) from the pulping process is combusted in order to transform cooking chemicals in the waste liquor into a form suitable for the recovery process. In a sulfate process the most important chemicals are sodium and sulfur. Organic substances dissolved in the waste liquor during digestion are combusted in the boiler producing heat which is used to convert the inorganic compounds contained in the waste liquor back into chemicals to be used in the digestion, and also for the production of steam. The inorganic matter of the waste liquor., i.e. ash, melts in the high temperature of the boiler and runs down as a primarily liquid smelt to the bottom of the furnace.
In conventional recovery boilers, the smelt is taken from the bottom of the boiler along one or more cooled smelt spouts to a dissolving tank. In the dissolving tank the smelt is dissolved into water or weak white liquor to produce soda lye, i.e. green liquor. The main components of the smelt, and the green liquor produced from it, in a sulfate process are sodium sulfide and sodium carbonate. The green liquor is then transported to a causticizing plant for white liquor production.
Hot smelt flow causes "banging" and explosions when it falls into the liquid in the dissolving tank. The "banging" noise is caused by the explosive reaction between the smelt and water when the smelt contacts the green liquor in the dissolving tank. The temperature of the smelt is on the order of 750-820.degree. C. and the temperature of the green liquor (or weak white liquor) in the dissolving tank, containing mainly water, is on the order of 70-100.degree. C.
The intensity of the explosive reactions in the dissolving tank may be controlled by breaking up the smelt flow running down the spout into small streams, droplets, flows, or pieces before it contacts the green liquor in the dissolving tank. In most cases, the smelt is disintegrated by directing a jet stream of green liquor against the smelt flow discharged from the spout. Also a jet of mist containing air and water has been suggested for this purpose. In Finland, the smelt is most commonly broken up by using low or medium pressure steam. However, the breaking up of the smelt flow using such disrupting streams is often uncertain and incomplete particularly when the flow path of the smelt between the smelt spout and the liquid surface in the dissolving tank varies. The flow path depends on the flow volume and the temperature of the smelt and other operating conditions, which inherently vary from time-to-time.
The conventional stationary breaking nozzle used for introducing disrupting media does not take into account the changing flow path of the smelt. It has been suggested that the position of the conventional breaking nozzle structure may be adjusted automatically, but such an automatic position adjustment causes problems. There is no reliable method available for adjusting the direction of the breaking nozzles with the change of the smelt flow path and the smelt volume. In practice the nozzle position (and disrupting stream direction) must be adjusted manually. However this is a dangerous job, and it is not economical or practical to assign a worker to perform this task on a continuous basis. The imperfect breaking up of the smelt pieces results in smaller or larger explosions which often cause an almost continuous loud noise in the vicinity of the dissolving tank. Liquid and smelt splashes and exhaust gases are usually prevented from escaping into the environment by a hood surrounding the smelt spout.
Usually, an excess volume of steam must be directed continuously to the smelt flow in order to secure adequate disintegration of the largest momentary smelt flow volumes. Thus, large volumes of steam may be wasted, which impairs the economy of the recovery process.
Swedish published patent application no. 381 295 discloses a method of improving the disintegration of smelt. The proposal in this reference is to cover the upper part of the dissolving tank with a semi-spherical hood having a wall thereof penetrated by the smelt spout of the recovery boiler. The smelt flow discharged from the spout hits a disc-like disrupting member inside the hood. The smelt may be further broken up by liquid jets disposed symmetrically below the disrupting member in the upper portion of the dissolving tank. A drawback of the disc-like disrupting member is that the sticky smelt may collect on its surfaces and consequently the disrupting capacity of the member is impaired. It must be cleaned manually, and the smelt lumps dislodged by cleaning fall into the dissolving tank, increasing the number of explosions. Because of the hot and chemically aggressive smelt, the life of such a disrupting member is very short even if it is made of fire-proof steel.
Japanese patent application no. 52-39575 discloses an apparatus for scattering and smashing the smelt flow. At first, the smelt arriving from the smelt spout is disrupted in an ordinary manner--by a steam jet disposed in the upper part of the dissolving tank--to produce coarse granules which then hit a cloud of water drops from horizontal nozzles, and after that impact an oblique plate before the fine smelt particles drop into the liquid in the tank.
The present invention seeks to improve the efficiency and reliability of the breakup of smelt discharged from the smelt spout of a recovery boiler to the dissolving tank and thus to minimize the number of explosive reactions between water and smelt compared to all of the prior art discussed above, in a relatively simple and inexpensive manner. The invention pays particular attention to the problems caused by variations in the smelt flow.
In order to achieve the advantages sought, the invention utilizes at least two dispersing nozzles distributing the same dispersing medium and disposed at a distance from the free end of tile spout and on different sides of a line parallel with the center axis of the spout. The nozzles are directed to apply at least one dispersing medium jet at a time obliquely downwardly to intersect the path of the smelt flow running down from the spout and thus to secure substantially uninterrupted breakup of the smelt flow during the process irrespective of the changes in the flow path.
According to the present invention, a new type of a smelt disrupting apparatus and method have been developed to reduce the noise problem of the dissolving tank. There are two or more dispersing nozzles provided in the vicinity of the smelt spout below the level of the end of the spout and directed towards the smelt flow from at least two different directions. At least two dispersing nozzles are located at the sides of the smelt flow running down from the smelt spout at a distance from a line parallel with the center line of the smelt spout (from the center line of the smelt flow). There may be one or more nozzles on both sides.
The dispersing nozzles may be arranged approximately at an equal distance from the free end of the smelt spout whereby the dispersing jets intersect the flow path of the smelt flow approximately at the same level. Preferably the dispersion nozzles are, however, disposed at different distances from the lower tip of the smelt spout. This arrangement guarantees that at least one dispersion jet hits the smelt flow from the side although the flow path of the smelt flow changes with relation to the dispersion nozzles due to smelt flow volumes and other process changes.
An additional nozzle or nozzles may also be arranged at other points by methods known per se. For example there may be an additional nozzle in front of the smelt flow and directed against the smelt flow or obliquely downwardly against the smelt flow. The nozzle may also be above the smelt flow directed approximately vertically downwardly.
According to one aspect of the present invention, an assembly for handling fluid smelt from a pulp mill recovery boiler is provided comprising the following components: A smelt spout connected to a recovery boiler, and having a free end tip, and a center line, fluid smelt downwardly flowing along the center line from said free end tip of said spout; and first and second nozzles for directing disrupting media toward the flowing smelt. The first and second nozzles are positioned on opposite sides of said center line, and so that at least one of the vertical and horizontal positions of said nozzles are different from each other so that the disrupting medium emanating from at least one of said nozzles intersects the smelt flowing downwardly from said smelt spout free end tip during normal operation even though the flow path of the smelt varies.
The first and second nozzles may each be spaced horizontally, in the dimension of smelt flow, from the spout free end tip a distance between about 200-500 mm, and the first nozzle is spaced a horizontal distance different from the second nozzle by between about 50-200 mm. Also, the first and second nozzles may both be positioned vertically below the spout free end tip a distance of between about 150-250 mm, and vertically spaced from each other a distance of at least 10 mm. For example, the first and second nozzles are each spaced from the spout free end tip a distance between about 300-700 mm, and at least 50 mm different than the other nozzle. Also, the first and second nozzles may each be spaced horizontally, in a dimension perpendicular to smelt flow, from the center line a distance between about 100-200 mm, and preferably the first and second nozzles are each positioned downwardly and obliquely at an angle to the vertical so that media emanating moves downwardly and obliquely making an angle of between about 20-40 degrees with respect to the vertical.
A third nozzle may be provided, e. g. positioned horizontally in front of the spout in the direction of smelt flow, and directing disrupting medium therefrom in a horizontal direction substantially opposite to the horizontal component of the smelt flow.
According to another aspect of the present invention, a chemical recovery assembly is provided comprising the following components: A pulp mill recovery boiler having a smelt spout extending outwardly from a bottom portion thereof. A green liquor dissolving tank positioned beneath the smelt spout to receive smelt from the smelt spout therein; the smelt spout having a free end tip, and a center line, fluid smelt downwardly flowing along the center line from the free end tip of the spout. And first and second nozzles for directing disrupting media toward the flowing smelt, the first and second nozzles positioned on opposite sides of the center line, and so that the first and second nozzles are each spaced horizontally, in the dimension of smelt flow, from the spout free end tip a distance between about 200-500 mm, and the first nozzle is spaced a horizontal distance different from the second nozzle bof between about 50-200 mm. Other details of nozzle positioning may be provided as described above.
According to another aspect of the invention there is provided a method of dissolving smelt to form green liquor while minimizing explosions as a result of hot (e. g. about 750-820 degrees C.) smelt contacting cool (e. g. about 70-100 degrees C.) liquid, using: a recovery boiler having a smelt spout extending outwardly from a bottom portion thereof; the smelt spout having a free end tip, and a center line, fluid smelt downwardly flowing along the center line from the free end tip of the spout; and a dissolving tank positioned below the spout and having cool liquid therein. The method comprises the following steps:
(a) causing fluid smelt to flow downwardly from the free end tip of the spout toward the dissolving tank, the smelt flow path inherently varying during normal operation of the recovery boiler; and
(b) before the smelt impacts the liquid in the dissolving tank, directing a disrupting medium in first and second distinct jets from opposite sides of the center line of the smelt flow toward the smelt flow to impact the smelt and break it into smaller flows, droplets, or pieces; and
wherein step (b) is practiced so that the distinct jets are differently directed so that, in combination, the disrupting medium from at least one of the jets intersects the smelt flowing downwardly from the smelt spout free end tip during normal operation even though the flow path of the smelt varies.
Step (b) is also preferably practiced so that jets are continuously emanating from both sides of the spout centerline at the same time (although based upon visual or automatic determination of the smelt flow at any particular point in time one of the jets may be turned off); and step (b) is also preferably practiced by positioning first and second nozzles which issue the jets of disrupting medium so that the first and second nozzles are each spaced from the spout free end tip a distance of between about 300-700 mm, but different by at least 50 mm from the other nozzle, and are positioned downwardly and obliquely at an angle to the vertical so that the jets move downwardly and obliquely making an angle of between about 20-40 degrees with respect to the vertical.
It is the primary object of the present invention to provide a simple yet effective method and apparatus for optimally disrupting the flow of smelt into a dissolving tank even when the smelt flow rate is inconsistent, varying during normal and inherent process changes. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.